The objectives of the research are to determine defense reactions of crustaceans to primary and challenge infections by a metazoan parasite. The crustacean hosts will be amphipods, Gammarus pseudolimneaus and G. minus, and the metazoan parasite an acanthocephalan, Pomphorhynchus bulbocolli. These host-parasite systems will be used as models from which resulting data may be applied to host-parasite systems in which the adult parasites ultimately parasitize humans. Defense reactions of the amphipods to initial infections by the natural route of ingestion of shelled acanthors and by direct inoculation of the acanthors into the haemocoel will be assessed by light microscope study of thin-sections of the amphipods stained with hematoxylin and eosin. Location of defense reactions (intestinal tissue and/or haemocoel), presence and extent of a haemocytic response, type of haemocyte involved, and the degree of melanization are the factors to be used in assessment of the defense reaction to a primary infection. Study of defense reactions to primary infections will be followed by study of defense reactions to challenge infections that are administered in the same manner and by reciprocal routes. Study of reactions to challenge infections will help to determine if defense reactions are accelerated during the challenge infection and if they occur during the migration of acanthors through intestinal tissue or involve a haemocytic response after the acanthors arrive in the haemocoel. Two species of host will be used to determine if physiological adaptation of the parasite to a host species plays a role in the defense reactions of the host.